Electronics manufacturers attach surface mount components to a circuit board by first printing solder paste on metallized areas on one or both major surfaces of the board. Once the paste is printed, surface mount components are placed so that each lead of a component contacts a corresponding solder paste-coated area. Thereafter, the circuit board is heated to reflow the solder paste, thereby bonding the component leads to the metallized areas.
During the component attachment process, different inspections are often performed. For instance, the board may be inspected after the solder paste is printed onto the metallized areas. The board also may be inspected after the components have been placed on the solder paste-coated metallized areas. Lastly, the board may be inspected after the solder paste has been reflowed to bond the components to the metallized areas. In the past, human operators performed each required inspection. Such inspections were found to be tedious and demanding, causing even the most careful operators to make mistakes during inspection.
In an effort to improve the process of circuit board inspection, automated inspection equipment has been developed. While such automated inspection equipment doesn't suffer from boredom and eyestrain like human operators, automated inspection equipment has been slow and expensive. More often than not, the accuracy achieved by current automated inspection equipment has not proven to be significantly better than the accuracy achieved by a human operator. Indeed, the human vision system is extremely capable of detecting irregularities in a two-dimensional or three-dimensional array of uniformly arranged and oriented items. As a human operator inspects an array of uniformly spaced items, the operator's eyes will focus quickly on those row(s) in the array containing a missing or misaligned item. Even if the array contains a large number of items, a human operator can easily detect those few that are missing and/or misaligned, provided there is large scale regularity.
Unfortunately, a typical circuit board does not have its metallized areas uniformly spaced and oriented in a two-dimensional array so as to possess large scale regularity. Most circuit boards have their metallized areas arranged in numerous individual gays. Consequently, the operator must shift his or her focus to each individual array, causing fatigue and eye strain. Thus, the arrangement of metallized areas on most circuit boards (as well as the arrangement of the component leads or pads attached thereto) is not consistent with an arrangement in which the error recognition capability of the human eye is optimized.
Thus, there is a need for an inspection technique that maximizes the capability of a human operator to detect defects.